A by-wire electric brake system calculates a target brake pressure using a driver input such as an amount of pedal stroke and generates a hydraulic pressure by various actuators, thereby achieving a target hydraulic pressure. In the electric brake system, electric motors are used as the actuators for generating a brake pressure.
Japanese Unexamined Patent Application Publication No. 2008-143419 describes a brake device for vehicles, the brake device including: a master cylinder that receives an input of a brake operation of an driver, and a slave cylinder as an electric brake device that generates a brake hydraulic pressure based on an electrical signal according to the brake operation. In the slave cylinder of the brake device for vehicles, when an electric motor is driven, a piston moves forward via gear mechanism, and a brake hydraulic pressure is thereby generated in a hydraulic pressure chamber which is formed in front of the piston, and the brake hydraulic pressure is outputted to a hydraulic path via a port. As the electric motor, for example, a three-phase AC brushless motor is used which has three winding wires of three phases: U phase, V phase, and W phase. In the three-phase AC brushless motor, the current value and phase of phase currents Iu, Iv, Iw are controlled, the phase currents being caused to flow through the winding wires by a drive circuit which is not illustrated.
However, in such a conventional electric brake device, while a brake pressure is controlled with an electric motor, the electric motor may be locked and a current may continue to flow through the same two phases consecutively. In this case, when two-phase continuously energized state occurs, in which, for example, the electric motor is not rotating or a current is flowing through two phases, the temperature of only two phases rises and a failure or the like of parts due to heat generation may occur. In addition, when a phenomenon occurs, for example, the pattern of the phase currents Iu, Iv, Iw becomes constant, heat generation may occur in the electric motor, the phase currents being drive currents of the electric motor.